A further three-day strike by Southern conductors is causing widespread disruption for travellers.
Members of the Rail Maritime and Transport Union (RMT) began the action at midnight.
Southern's parent firm Govia Thameslink (GTR) is in dispute with the RMT and Aslef unions over changes to the role of guards on new trains.
The RMT is staging the latest 72-hour walkout after months of strikes. Another strike is set for 9 January.
The train drivers' union Aslef is continuing with a ban on overtime which is adding to the disruption.
GTR has urged people to check travel information because some New Year's Eve services will finish early.
Southern's deputy chief operating officer, Alex Foulds, said: "While we will do everything we can to get passengers where they want to go, we have to warn people that journeys may be disrupted and take longer than normal.
"We are sorry that yet again passengers will have their plans disrupted by what is pointless industrial action. Our door remains open for meaningful talks."
The planned six-day strike starting on 9 January involves both the unions. Most routes are expected to have no services.
RMT general secretary Mick Cash said: "Our guards members on Southern Rail remain rock-solid and absolutely determined in their action in defence of rail safety this morning."
The urged transport ministers to engage in direct talks with the union.
The Department for Transport said the dispute was between Southern and the unions and "not something the government is involved in".
The RMT has previously accused Southern of "hoarding profits" but the rail operator said it made a loss last year and would not make any profit this year.
During the last drivers' strike, Aslef went into talks with GTR at conciliation service Acas. Mr Cash said he was not allowed in.
The negotiations ended without a deal.
Commuter Robin Marchant, from Brighton, has organised a mass peaceful protest for passengers from 3 January to 5 January.
So far more than 1,000 people have shown interest in the Facebook event.